The present invention relates to a method for producing a ceramic composite. In particular, the present invention relates to melt-infiltrated ceramic composites and to a method for producing the same.
U.S. Pat. Nos. 4,120731; 4,141,948; 4,148,89; 4,220,455; 4,238,433; 4,240,835; 4,242,106; 4,247,304; 4,353,953; 4,626,516; 4,889,686; and 4,944,904; assigned to the assignee hereof and incorporated herein by reference, disclose molten silicon infiltration of materials which include carbon, molybdenum, carbon-coated diamond, cubic boron nitride, and blends of carbon with silicon carbide, boron nitride, silicon nitride, aluminum oxide, magnesium oxide and zirconium oxide.
Reinforced ceramic matrix composites (xe2x80x9cCMCsxe2x80x9d) comprising micron-sized fibers having one composition, which fibers are dispersed in continuous ceramic matrices of the same or different composition, are well suited for structural applications because of their toughness, thermal resistance, high-temperature strength, and chemical stability. Such composites typically have high strength-to-weight ratio that renders them attractive in applications in which weight is a concern, such as in aeronautic applications. Their stability at high temperatures renders them very suitable in applications in which the components are in contact with a high-temperature gas, such as in gas turbine engine. For many of these components, such as a gas turbine shroud, the dimensional and shape tolerances that must be met by the composite components are very tight.
The production of CMCs typically begins with providing a fiber preform which is a porous shaped object made of micron-sized fibers with a protective coating, a portion of the matrix material supplied either as particulates from a ceramic precursor or by chemical vapor infiltration (xe2x80x9cCVIxe2x80x9d), and an amount of temporary binder. The fibers of the preform have typically been woven previously into a cloth or otherwise assembled into a structure. The porosity within the fiber preform is then filled with a matrix precursor material, often a molten metal such as silicon, that eventually produces the finished continuous ceramic matrix surrounding the fibers. SiC fibers have been used as a reinforcing material for ceramics such as SiC, TiC, Si3 N4, or Al2O3. The filling of the fiber preform with the matrix precursor material and any attendant reaction between the matrix constituents already in the preform and the precursor material serve to densify the shaped object. This filling or densification may be achieved by chemical-vapor or liquid-phase infiltration by the matrix precursor material. Liquid-phase infiltration, often by a molten metal, is the preferred method because it is less time consuming and more often produces a fully dense body than the CVI process. Full densification is necessary to achieve good thermal and mechanical properties and, thus, a long-term performance of CMCs. However, liquid-phase infiltration also has shortcomings. First, it is usually conducted at a temperature only slightly above the melting point of the matrix precursor material and for as short a time as possible to prevent the undesired rapid deterioration of the fibers. This can result in nonuniform infiltration characterized by incompletely filled areas and, thus, an inferior product. Second, in order to ensure a more complete infiltration, a larger amount of metal infiltrant material than required is supplied on the surface of the preform with the result of having excess metal infiltrant remaining on the surface of the composite when the infiltration process is terminated. Although post-infiltration machining of this excess metal can be undertaken, it not only increases the fabrication cost but also can accidentally expose some of the fibers and provide points for environmental attack. Third, during the liquid-phase infiltration, especially with molten silicon, the shaped body typically passes through a relatively weak state (following burn-out of the temporary binder but before a complete infiltration and reaction of the matrix material) which can easily yield a distorted finished shape.
Therefore, it is very desirable to provide a liquid-phase infiltration process that can produce a shaped CMC article having uniform distribution of continuous matrix phase in a short time. It is also very desirable to provide a liquid-phase infiltration process that substantially preserves the dimensions of the article throughout the fabrication process and that does not require substantial further machining of the final densified article.
The present invention provides a method for producing shaped articles of melt-infiltrated ceramic composites, which method overcomes many shortcomings of the prior-art liquid-phase infiltration processes. The method of the present invention comprises the steps of: (1) providing a stable formed support; (2) disposing a fiber preform adjacent to the stable formed support; (3) providing at least a precursor of the ceramic matrix materials to the fiber preform; (4) heating the fiber preform with at least a precursor of the ceramic matrix materials still disposed on the stable formed support to a temperature greater than or equal to the melting point of the precursor of the ceramic matrix materials; (5) allowing the precursor in a molten state to infiltrate the fiber preform to result in a melt-infiltrated composite; and (6) cooling the melt-infiltrated composite to result in the shaped article of melt-infiltrated ceramic composite. The stable formed support may comprise a material that is not attacked by precursors of ceramic matrix materials at the infiltration temperature. Alternatively, the surface of the formed support may be coated with a material that protects the formed support from being attacked by precursors of ceramic matrix materials. As used herein, a xe2x80x9cstable formed supportxe2x80x9d is one that remains a solid throughout the fabrication of the shaped articles and does not exhibit any creep or plastic deformation, and the dimension of which does not change by more than the dimensional tolerances required in the finished shaped CMC articles.
The surface of the stable formed support adjacent to the fiber preform is provided with a plurality of indentations that extend beyond at least one end of the fiber preform. In one aspect of the present invention, the indentations are grooves, slots, or channels formed into the surface of the stable formed support. These indentations may also be interconnected.
In another aspect of the invention, the stable formed support is a hollow structure having an outside cross-sectional dimension substantially equal to an inside cross-sectional dimension of the fiber preform. The stable formed support has a plurality of indentations formed on the surface adjacent to the fiber preform.
In still another aspect of the present invention, the stable formed support with the fiber preform and the precursor ceramic matrix material provided thereon is disposed within a heating device, and a vacuum is applied to the interior of heating device. The stable formed support with the fiber preform and the precursor ceramic matrix material provided thereon is heated to at least the melting point of the precursor of the ceramic matrix material so that it melts and flows along the fiber preform surface adjacent to the plurality of indentations, thereby infiltrating the fiber preform from both sides.